This invention relates to the casting of metals. More particularly, it relates to the continuous or semicontinuous casting of aluminum-lithium alloys. Certain metals, such as aluminum alloys containing more than about 0.25% lithium, are highly reactive when exposed to certain environments. Lithium metal being an alkali metal, reacts vigorously with water such as in a DC permanent fixed mold type or electromagnetic type continuous or semicontinuous casting operation and where the lithium metal is in a molten state. Aluminum-lithium alloys present other unusual problems such as oxidation at elevated temperatures in the solid or liquid state. This means that potentially serious explosion hazards are present when the aforesaid casting procedures are employed with aluminum-lithium alloys. Explosive reactions involve great releases of energy and potentially severe damage to equipment and injuries to personnel.
While these dangerous conditions can be controlled through the use of expensive equipment and processes including inert gases, oxygen free atmospheres, and vacuum induction melting furnaces that afford controlled atmospheres and environments, the costs have been prohibitive or excessively high. Thus these explosive hazards and cost problems have hampered and delayed the large-scale commercial production and exploitation of aluminum-lithium alloys despite the many advantages of the same in finished product applications. One of the most desirable applications for aluminum-lithium alloys involves aircraft components because of a lower density and higher modulus than the standard high-strength heat-treatable aluminum alloys that are presently available.
The nature of the DC tubular fixed mold and/or electromagnetic casting processes requires precise control of the many elements involved, such as mold type; casting or drop rate; mold lubricant; ingot size; liquid or molten metal distribution within the confines of the mold; water quantity and temperature; and liquid metal heights within the mold. When control of one or more of these variables is interfered with or lost during a casting operation, various problems, such as "bleedouts", can occur. A "bleedout" is a phenomenon wherein a flow of molten metal takes place along an already solidified outer surface of an ingot much in the same fashion as the wax flows down the side of a candle. As indicated in U.S. Pat. No. 2,983,972, a "bleedout" can be the result of a localized remelting through the initially thin-chilled wall of the solidifying ingot caused by the heat of the hot molten metal in the inside of the ingot probably under the influence of pressure from the hydrostatic head of molten metal at the top of the ingot. If a "bleedout" occurs, hot liquid metal will come into direct contact with the usual water cooling medium. When aluminum-lithium alloys are involved, the result can be explosively dangerous and disastrous. The loss of electrical power or a disruption of the coolant water pattern on the mold and/or ingot at the casting stations presents "bleedout" opportunities. Further complicating the control elements and the propensity for "bleedouts" to occur is when the alloy being cast has a relatively low solidus line temperature, which is approximately 900.degree. F. (482.2.degree. C.) for the highly alloyed lithium-containing aluminum alloys.
Accordingly, it is a primary purpose of this invention to provide improved processes and systems for continuously or semicontinuously casting lithium-containing alloys selected from the group consisting of aluminum and the alloys thereof, and wherein lithium is one of the constituents and preferably a major constituent. For the purposes of this invention, a clad cast lithium alloy as described and claimed herein shall be a metal alloy, and preferably, an aluminum alloy containing more than about 0.25% lithium as a constituent along with the usual impurities.
Examples of the types of aluminum-lithium alloys that may be clad cast by the processes and systems of the invention are those described in European Patent Application Nos. 090,583 and 088,511 and United Kingdom Patent Application Nos. 2,115,836A, 2,127,847 and 2,121,822A. A further aluminum-lithium alloy could be one containing up to 2.5% Li; 1.2% Cu; 0.7% Mg; 0.12% Zr and the balance aluminum.
It is a further purpose of this invention to cast in a substantially continuous or semicontinuous fashion aluminum-lithium-containing alloys within a precast solid metal shell, and preferably an aluminum alloy shell having a higher solidus line temperature than that of the aluminum-lithium alloy in order to prevent direct contact between the aluminum-lithium alloy and the coolant and thus avoid occurrence of a "bleedout" of the encircled or encased aluminum-lithium alloy. In effect, the aluminum-lithium alloy is cast within an outer protective aluminum mold or shell with the coolant medium being applied directly to the outer aluminum mold. The resultant product is a structurally sound and composite ingot, billet or like article made up of a first metal structural component or core material, i.e., the aluminum-lithium alloy and a second metal structural component which is the protective outer metal shell or mold, preferably in the form of an aluminum alloy such as an aluminum alloy of the 1000 series as designated by the Aluminum Association in the United States from which lithium is absent as a constituent or usual impurity although it may be tolerated as a trace element.
Another attribute of this improved casting system and method is the uniqueness of continuously casting the outer protective mold itself substantially simultaneously with the encased aluminum-lithium alloy core. This permits more precise control of various metallurgical conditions while providing savings in time, equipment, and personnel. For example, this allows a simultaneous spreading or wetting by the liquid portion of the core alloy of the inner surface of the solid mold or outer alloy and a continuous and integral metallurgical interface and bonding therebetween throughout substantially the entire casting operation. This in turn means that the desirable heat transfer features of the direct chill process for both the core material and outer alloy mold will be retained while the core alloy solidifies.
In a further advantageous embodiment of the invention and in order to compensate for thermal contraction or shrinkage of the outer cladding material and mold and yet ensure free movement or passage of the cladding out of the main mold unit to form in and of itself a fully solidified combination cladding and casting mold of the desired and substantially uniform cross-sectional thickness or width at the plane or point of initial contact between molten metal core and outer metal cladding mold, components of the main casting mold can be somewhat tapered to provide a somewhat larger mold space in cross section at the exit end thereof.
The improved process and system advantageously allow the subsequent fabrication and processing of the final composite or metal clad ingot or encased article by way of conventional rolling, extruding, and forging equipment, etc. It is well known and as indicated in the aforesaid United Kingdom Patent Application No. 2,121,822A, that elevated temperatures such as occur during homogenizing, annealing, solution heat treating, and hotworking, can have serious detrimental effects relative to the surface of aluminum-lithium alloys because of the oxidation activity and lithium losses due to such oxidation.
In the past, where lithium alloys were handled, it was usually necessary to have controlled atmospheres where oxygen and water vapor were excluded by appropriate enclosures such as are indicated in U.S. Pat. Nos. 3,498,832, 3,368,607 and 4,248,630. With the composite or clad alloy ingot obtainable by practice of the invention, a protective environment is not needed as the ingot exits the segmented or dual mold of the instant invention. The clad product in effect carries its own protective environment by way of the cladding. On the other hand, in those instances where the cladding material resulting from the casting operation is not desired in the final product but only the aluminum-lithium core alloy itself, the cladding can be readily removed by standard scalping or cutting tools, sanding, or chemical etching.
Additional features of the improved casting process and system of the instant invention involve the unique metallurgical bonding occurring between the mold and core alloy whereby the properties of a wrought-type product can be obtained, plus the ability to reduce or eliminate the cracking propensity of many alloys, especially the more highly alloyed, solution heat-treatable-type alloys involving lithium as a constituent. The precast solid aluminum shell forming the outer common casting mold and cladding of the final product advantageously retards the thermal shock incurred when such alloys are cast in the conventional DC casting operations using either a fixed mold or electromagnetic equipment. This thermal shock affects both the physical changes that take place during solidification, such as the 6% to 8% volumetric change, as well as the changes associated with a simultaneous solution heat-treat effect, as the metal moves from the liquid to solid phase and a rapid chilling of the casting takes place. In the past, this superimposition of the physical and metallurgical changes or phenomena created enormous stress areas in an ingot which often resulted in spontaneous stress relief by the physical cracking during casting or later rolling and ultimate scrapping of the ingot products. It is to be further understood that in the practice of the invention, the size and shape of a given ingot, as well as the thickness of a given cladding, plus the speed of casting, i.e., drop rates, coolant and contraction rates, etc., will all depend on the specific results and products desired, provided, of course, the cladding is sufficiently solidified and of sufficient thickness at the point or plane of initial and subsequent cladding and core contact to withstand the metal head and pressures of the molten metal core.
Although as noted the thickness of the cladding material will vary with individual requirements for the structurally composite ingots one preferred embodiment of the invention contemplates that the present specificiations for alclad sheet and plate be used. Accordingly, the cladding thickness can range from 1.5% to 5%.+-. casting tolerances of the total ingot thickness per side for a non circular ingot or of the diameter for a circular ingot or billet. Thus, if a rectangular in cross section ingot has an overall thickness of 20" (50.80 cm) each side cladding should be between 0.3" (0.762 cm) and 1" (2.54 cm) in the case of a billet 20" (50.80 cm) in diameter between 0.3" (0.762 cm) and 1" (2.54 cm).
Various schemes have been proposed in the past involving continuous or semicontinuously direct chill or DC tubular mold or similar casting operations for producing clad and composite ingots, billets, or like articles, including those made from aluminum alloys, as indicated, for example, in U.S. Pat. Nos. 3,206,808, 3,353,934, 3,421,569, 2,055,980, 3,421,571 and 4,213,558. Further, prior art segmented or multiple mold clad casting equipment is disclosed in U.S. Pat. No. 2,264,457, German Pat. No. 844,806, and at pages 277-280 of the Handbook of Casting by Dr. Erhard Herrman (Handbuch des Stranggeissen), Copyright 1958 by Aluminium-Verlag GmbH. None of these patents, however, as well as the literature reference, recognizes the advantages or concepts of such practices as applied to the economical large-scale production of aluminum-lithium alloys.
One final observation is believed to be in order regarding prior art continuous clad casting processes such as is disclosed in U.S. Pat. No. 3,470,393, wherein a cladding material is cast about a solid core prior to reviewing the details of the instant process and system. The instant development proposes a basically reverse-type concept in that it contemplates solidifying the cladding metal first rather than the core metal so that the cladding can advantageously form a solid outer impervious tubular casting mold or envelope that can be filled with a molten aluminum-lithium alloy and not vice versa.
In the claims and detailed description of the invention which is to follow, the term "tubular cladding and casting mold" is meant to cover a combination outer protective sheath or envelope and mold for an inner metal core containing lithium wherein the metal core is in intimate metallurgical contact with the aforesaid outer cladding sheath. The combination cladding and moving casting mold is preferably formed by direct chill or DC casting using a fixed or permanent tubular mold, assembly or an electromagnetic inductor and appropriate associated mold elements. While the ensuing discussion of the various embodiments of the invention will be directed to DC casting operations of the aforesaid types it is believed that the teachings of the invention can be extended to rotating casting wheels and cooperating belt means or a pair of moving cooperating belts. The term "tubular" is meant to include any shape that had an endless geometric outer surface or peripheral configuration in cross section. Thus, the basic casting mold arrangement for the final product can be circular, rectangular, square, elliptical, hexagonal, etc.